In filter testing, filters are subjected to a so-called pressure-resisting test at a predetermined pressure by a testing device. The sealing of the filter, which has previously been wetted with a liquid such as water, is tested by the presence or absence of air leakage therethrough. The filter is subsequently subjected to a test for determining its so-called "bubble point". Upon reaching the bubble point pressure, the pressurized air will flow freely through the filter. This pressure measured upon reaching the bubble point is in effect a measurement for pore size, according to which filters are classified. Also, when a filter is dirty, one can determine the degree of dirt present, since partially clogged pores increase the pressure of the bubble point.
A testing device of the above-mentioned type is known from German GM No. 79 31 402 and includes a testing apparatus which is partially filled with water and is connected to the air outlet of the filter housing during testing of a filter. An inlet valve is manually opened until the appropriate pressure-resisting test pressure is reached, which pressure can be read from a manometer which is provided on the filter housing. Based on air bubbles which rise from the liquid upon reaching of the pressure-resisting test pressure, conclusions are supposed to be drawn concerning the sealing of the filter. This optical testing method is complicated, time consuming and inexact. Thereafter, through further opening of the inlet valve, which is supposed to be carried out slowly and evenly, the air pressure is increased until the operator recognizes the "bubble point" through increased air bubble formation in the testing apparatus, and then the associated pressure must be read from the manometer. This is also complicated, time consuming and inexact.
It has also been suggested that the liquid reservoir of the testing apparatus can be constructed as a venturi pipe, which in its constricted area has electrodes which are connected to an ohmmeter and measure the conductivity of the water in the liquid reservoir. The conductivity of the water in the liquid reservoir is relatively high in the absence of bubble formation, while increasing bubble activity causes the conductivity to drop, the increasing resistance being readable on the ohmmeter. The ohmmeter is preferably coupled with a recording mechanism, for example a recorder which graphically registers conductivity and thus bubble activity. This method of measuring bubble activity is also inexact and has not been particularly successful in practice, because the exact bubble point pressure can only be determined by visually reading the manometer.
Problems also arise when, according to known testing methods, sterile filters must be tested. Since testing takes place at the outlet of the filter housing, and thus at the sterile side, the danger of a secondary contamination exists. To avoid this secondary contamination, the known testing devices are equipped with additional filter surfaces, which are constructed partly moisture-repellent and partly hydrophilic. This results in disadvantages, including higher manufacturing expenses.
One basic purpose of the invention is therefore to provide a filter testing device of the above-mentioned type, particularly for sterile filters, which operates precisely, quickly, and substantially automatically, which provides reproducible results, thereby permitting a precise recording of the pressure-resisting test pressure and the bubble point pressure, and which, in the case of a sterile filter, avoids in a simple manner any secondary contamination.